1. Field
The present invention relates to a process for preparing diene elastomers, such as butadiene or isoprene homopolymers or copolymers. The invention especially applies to the production of a polybutadiene with a high level of cis-1,4 linkages that has at the same time a Mooney viscosity ML(1+4) greater than or equal to 40, a polydispersity index less than 2.3 and a reduced cold flow, preferably less than 0.3.
2. Description of Related Art
In order to prepare butadiene or isoprene homopolymers or copolymers having a high level of cis-1,4 linkages, it is known to use catalyst systems based on:                a salt of a rare-earth element in solution in a hydrocarbon-based solvent;        an alkylating agent of this salt formed from an alkylaluminium; and        an alkylaluminium halide.        
Thus, Patent Document RU 2,139,298 C1 teaches a process for polymerization of cis-1,4-polyisoprene and polybutadiene by polymerization of the diene using a “preformed” catalyst system comprising 3 constituents:                (a) a rare-earth carboxylate;        (b) an alkylaluminium sesquichloride; and        (c) a non-halogenated organometallic aluminium compound.        
The non-halogenated organometallic aluminium compound is added either before, or after the other constituents a) and b) of the catalyst.
This separate introduction of the constituents c) relative to the constituents a) and b), by varying the amount introduced and the organo Al/rare earth molar ratio, makes it possible on the one hand to reduce the amounts of rare-earth catalyst and of non-halogenated organometallic aluminium agent required and on the other hand to confer a certain flexibility to the polymerization process. Polymerization in a battery of reactors makes it possible to regulate the characteristics of the targeted polymers and, to a certain extent, to improve the cold flow which remains, however, detrimental from an industrial processing point of view for the polymers thus obtained.
Patent document EP B 1 055 659 teaches, in order to obtain a polybutadiene having a high level of cis-1,4 units, the use of a catalyst system having three constituents:
(a) a neodymium neododecanoate;
(b) an alkylaluminium compound or the corresponding hydride; and
(c) an organic halide,
optionally in the presence of an alkylaluminium as a chain modifier by way of its transfer agent properties that regulates the molecular weight.
Patent document JP-A-59-45 311 describes a process for preparing (in 2 phases) polybutadiene having a level of cis-1,4 units greater than 70% using a catalyst based on rare-earth elements from the family of lanthanides, an alkylaluminium compound as an alkylating agent and an alkylaluminium halide, the first phase consisting in polymerizing the butadiene in the presence of the above catalyst and the second phase consisting in adding, after 3% conversion of the monomer, an aluminium hydride. This separate addition of the aluminium hydride makes it possible to improve certain properties of the polybutadiene due to the possibility of controlling, to a certain extent, the molecular weight distribution and in particular of widening the molecular weight distribution.
Patent document EP-B 207 558 teaches, in order to obtain a butadiene homopolymer or copolymer having a Mooney viscosity that is more or less independent of the degree of conversion of the monomers:                initiating the polymerization reaction with a catalyst system comprising:                    a) a neodymium compound chosen from neodymium oxide, and neodymium alkoxides, phenates and carboxylates;            b) an organic compound having a hydroxyl or carboxyl group;            c) a non-halogenated organometallic aluminium compound; and            d) a halogenated compound chosen from secondary or tertiary alkyl halides, organic acid halides, metal and organometallic halides, hydrogen halide acids and halogens,                        
so that the aluminium/neodymium molar ratio in this catalyst system varies from 10 to 30; and                adding in continuous or batch mode, after the start of the polymerization reaction and over a time period at least equal to half of the total duration of this reaction, a given amount of this compound c) until an aluminium/neodymium molar ratio ranging from 20 to 80 is obtained.        
As indicated in the exemplary embodiments of this document, the purpose of this addition of the organoaluminium compound after the initiation of the polymerization reaction is to use its transfer agent properties regulating the molecular weight and thus to obtain Mooney viscosities ML (1+4) which are approximately constant and below the Mooney viscosities which are obtained by adding this compound at the same time as the catalyst system, that is to say less than or equal to 40, which does not make these polybutadienes well suited for use in tyre cover treads.
Patent documents WO-A-02/38636 and WO-A-03/097708 in the name of the Applicants teach, in order to obtain polybutadienes and polyisoprenes, to use a “preformed” type catalyst system based on at least:                one preforming conjugated diene, such as butadiene;        one organic phosphoric acid salt of one or more rare-earth metals, which is in suspension and in at least one saturated and aliphatic or alicyclic, and inert hydrocarbon-based solvent;        one alkylating agent composed of an alkyl aluminium of formula AlR3 or HAlR2 in which R represents an alkyl radical, preferably of 1 to 8 carbon atoms and H a hydrogen atom; and        a halogen donor which belongs to the family of alkylaluminium halides, with the exclusion of alkylaluminium sesquihalides.        
The polybutadienes obtained by means of this catalyst system have, in particular, a polydispersity index less than 2.1 and a Mooney viscosity ML(1+4) at 100° C. which may be within a relatively wide range of values, of around 25 to 80. These combined characteristics make these polybutadienes well suited for use in tyre cover treads.
The polyisoprenes as obtained by means of this catalyst system have, in particular, a polydispersity index less than 2.30 and a Mooney viscosity ML(1+4) at 100° C. which may be within a relatively wide range of values, of around 40 to 100. These combined characteristics make these polyisoprenes well suited for use in tyre cover treads.
Two weaknesses exist however for this catalyst system. Firstly, the synthesis is carried out batch mode but furthermore, the relative proportions of the four constituents are fixed as a function of the diene monomer(s) to be polymerized, the desired characteristics for the expected polymer, the polymerization process (residence time and temperature imposed by the material for example) and the level of impurities present in the industrial unit used to carry out the polymerization. It is thus easy to understand, for a person skilled in the art, that these last two elements (synthesis of the catalyst in batch mode and formulating depending on 4 parameters) result in a lack of flexibility of the catalyst system and therefore of the polymerization process, especially in the context of application on an industrial scale.